U.S. Pat. No. 6,240,135 teaches a method of encoding moving pictures using a plurality of blocks. This method includes a step of defining pixel sets S0, S1, S2 around a block boundary, a step of selectively determining a deblocking mode as a default mode or a DC offset mode depending on the degree of block noise after acquiring a mode decision value, a step of obtaining frequency information around the block boundary per pixel using a 4-point DCT kernel if the default mode is selected, a step of replacing a magnitude of a discontinuous component belonging to the block boundary with a minimum magnitude of discontinuous components belonging to the surroundings of the block boundary in the frequency domain, a step of applying the replacing step to the spatial domain, a step of deciding if it is necessary to execute the DC offset mode if the DC offset mode is selected, and a step of removing the block noise in a smooth region if executing the DC offset mode is decided to be necessary. A flow chart of this method of the related art is shown in FIG. 1.
This method of the related art uses two modes, the default mode and the DC offset mode, to adjust the pixel values causing block noise. In the default mode frequency information for each pixel near the block boundary is acquired using a 4-point DCT kernel. The magnitude of a discontinuous component belonging to the block boundary is replaced with a minimum magnitude of discontinuous components belonging to the surroundings of the block boundary in the frequency domain. Low-pass filtering is applied in the DC offset mode. Only the boundary pixel values are corrected in the default mode. The default mode is thus not sufficient for removing block noise in extremely smooth regions, such as settings in photographs. Block noises in smooth regions are therefore removed in the DC offset mode.
This method of the related art applies a DCT-based filter or a low-pass filter to pixels along the block boundary to smoothen discontinuities caused by extremely smooth regions. The low-pass filter is used in extremely smooth regions, and the DCT-based filter is used in other regions. Because differences between pixels surrounding the block boundary are reduced by using these filters, discontinuity is reduced and block noise can be removed. However, the DCT-based filter or low-pass filter not only reduce abnormal discontinuities, it also removes natural changes between pixels. Image definition is lost as a result of removing natural changes between pixels. This method of the related art removes block noise and poor definition using a DCT-based filter or low-pass filter. As a result, the value of the image also drops.
Block noises are produced at the block boundary when the pixel values along the block boundary create abnormal discontinuities. The method of the related art applies a DCT-based filter or low-pass filter to pixels along the block boundary to smooth the discontinuities. However, the DCT-based filter or low-pass filter not only reduces abnormal discontinuities, it also removes natural changes between pixels. The conventional method of removing block noise results in a poorly defined image and image quality drops.